CINCINNATI — Take a hazardous waste dump contaminated with the deadly herbicide Agent Orange. Throw some hungry bacteria on it and come back in a few months to find the chemical devoured, reduced to table scraps of carbon dioxide, water and salt.

Once a futuristic concept, this technology is about to jump from the petri dish to the forefront of the nation's battle against toxic chemicals.

Scientists will start field tests in 1986, possibly in Florida. They predict that within five to 10 years, natural and genetically engineered bacteria will revolutionize the treatment of hazardous waste.

The U.S. Environmental Protection Agency is understandably excited. The agency lists 812 contaminated sites eligible for cleanup under the federal Superfund program. The congressional Office of Technology Assessment estimates that up to 10,000 chemical dumps may need to be cleaned at a cost of $100 billion.

Bacteria may work more cheaply and efficiently than current cleanup methods. EPA officials talk of building underground barriers around dump sites to prevent contamination from spreading, giving scientists time to develop the right bacteria to eradicate dangerous chemicals.

At EPA and university labs, bacteria are being cloned, engineered, mutated and studied down to the microscopic genes that form the basis of all life.

''It is amazing the things we are finding out every day,'' said Al Bourquin, an EPA microbiologist in the Florida Panhandle town of Gulf Breeze. ''The benefit is just going to be astronomical.''

Florida, which has 120 sites with contaminated groundwater, is a good candidate for such technology because the state's warm climate provides fertile ground for bacteria.

Critics say genetically engineered bacteria strains could prove more dangerous than the chemicals they clean up.

Bacteria form the bottom rung of the food chain, and tinkering with this system invites catastrophe, said Jeremy Rifkin, an environmental activist in Washington and the nation's most outspoken critic of genetic engineering. Man could be creating a living pollution, he warns, one that multiplies and spreads.

Microbiologists say the risks can be controlled, that the bacteria can be designed to eat their designated dinners and disappear. They also argue that many of the bacteria being studied are found naturally in the environment.

''We are not worried that it will become the monster that ate Orlando,'' said Dr. Pat Sferra, an EPA biologist in Cincinnati.

When something decays, bacteria actually are eating it or breaking it down. Most toxic chemicals are made of organic compounds, and likewise can be broken down. In some cases, bacteria eat a chemical and in others they rearrange its atoms to make it less toxic.

However, chemical companies have made compounds that bacteria find hard to swallow, allowing chemicals to remain in the environment for years. The pesticides DDT and EDB (ethylene dibromide) are the best examples.

These and other chemicals can seep into underground reservoirs, or aquifers, where there are few bacteria to break them down. The EPA and Congress are phasing out burial of wastes in landfills because this leaves the chemicals intact, ready to haunt future generations.

Pumping and treating contaminated groundwater is expensive and time consuming. In some cases this puts the chemicals in the atmosphere rather than destroying them.

Bacteria, however, often break down chemicals into harmless products such as carbon dioxide. And, as David Stephan, director of EPA's hazardous waste engineering laboratory in Cincinnati said, ''When you get bugs working for you, they work cheap.''

Bacteria are being recruited from three places: nature, the laboratory and what is best described as somewhere in between.

In nature, bacteria abound that can eat almost anything people throw away. The right bacteria, however, may not be where the contamination is.

''Most bugs we need probably exist in nature, just not at the right place at the right time,'' said Sferra.

Next year the EPA may transplant the white rot fungi bacteria to Eglin Air Force Base near Pensacola to test its ability to feed on Agent Orange dumped there years ago, Sferra said.

In some cases, bacteria may live at a toxic dump, but not in large enough numbers to dent the contamination. So scientists hope to make them reproduce faster.

Scientists at Stanford University in California are pumping oxygen and nutrients into an aquifer contaminated with organic solvents, hoping to increase a natural population of organisms that can break down the chemicals. The test will end within a year. If it works, similar tests may be done in Florida, said Bourquin of the Gulf Breeze EPA lab.

Bacteria from genetic engineering are the most controversial. Scientists construct them by slicing out genes from one cell and transplanting them into another.